Someone told me that if you ask for a certificate at the clinic, they will give you one is this true?
"It depends". Some will, and some of those will charge for them.
The big reason for not handing them out generally - and not relying on any that you see without being able to trust the person clutching one, thus ruling out the vast majority of sex work situations - is that they only tell you something about the past.
So for HIV, the usual test will be able to say 'this person very probably didn't have HIV about a month ago'. Anything could have happened in the meantime and if they have caught HIV then, they will be particularly infectious despite having the piece of paper saying the test was HIV-.
There's also no standard format, so it would be easy for someone to run something official looking up themselves.